Japanese Patent No. 2816698 discloses a stator for motors that comprises a stator core including a substantially annular yoke and a magnetic pole constituent member arranged inside the annular yoke, and an excitation winding section arranged at a magnetic pole constituent member. The magnetic pole constituent member integrally includes a plurality of pole columns, each of which has one end connected to an inner surface of the yoke, magnetic pole surface constituent sections respectively arranged on the other ends of the plurality of pole columns, and connecting portions connecting two adjoining magnetic pole surface constituent sections. The yoke and the magnetic pole constituent member are respectively formed by overlaying a plurality of magnetic steel plates. In the inner peripheral surface of the yoke, a plurality of fitted recesses, which are open toward both sides of the overlaying direction of the magnetic steel plates and also open toward the magnetic pole constituent members, are formed. The one ends of the pole columns are fitted into these fitted recesses. This arrangement is shown in FIGS. 1 to 4 of Japanese Patent No. 2816698.
In a conventional stator for motor, however, the inner peripheral surface of the annular yoke is curved in a form of an arc. Thereby, a lateral cross-sectional shape of a space, which is formed on both sides of the pole column for receiving an excitation winding section, is a fan in shape. As a result, a configuration of the excitation winding section is restricted by the configuration of this space, and there has been a limitation in increasing a space volume occupied by the winding wire of the excitation winding section.